The invention relates to the compensation of variable velocity in scanners used for image analysis, and particularly optical lecture, such scanners comprising means for producing a video signal indicative of the light intensity received at successive regular time intervals from an image scanned along a first direction at an automatically controlled known motion velocity and along a second direction at an unknown motion velocity.
Applications for optical scanning are known in which one of the scan directions is supplied by the relative motion between a scanning head and the scanned medium. Where the relative velocity is fixed and known, the scanning head may be in the form of a linear array of photodetectors on which a narrow slit-shaped portion of the scanned medium is focussed, the long axis of the array being perpendicular to the direction of the relative motion. The sampling rate of the array is controlled by a clock pulse in the well known manner at a frequency such that successive column starts of the scan occur when the incremental distance travelled by the head relative to the scanned medium is one notional column of the discrete raster into which the image of the scanned medium is decomposed. However, when the relative velocity is unknown, as is the case, for example, with handhold scanners (sometimes called optical wands), this technique will not yield a useful raster since it will result in distorted images.
A known technique for rendering optical wands independent of the relative velocity of the scanning head with respect to the medium uses a two-dimensional (rectangular) array of photodetectors. The rate of sampling of the two-dimensional array is made sufficiently high that the entire array is sampled several times during the passage of the array over a single printed symbol to be analysed, and logic means are supplied for selecting one, best image among the multiple set of images obtained for each single scanned symbol. This technique presents multiple disadvantages. Among these are the following:
1. The rate of sampling must be exceedingly high, resulting in excessive signal-handling requirements and reduced signal/noise ratio; PA1 2. Even at the highest sampling rates the array size is strictly limited by this speed requirement, so that either the height of the array or the width is insufficient, or both; insufficient height leads to excessively tight requirements on the vertical position of the scanner with respect to the scanned line of print, while insufficient width restricts the range of applications of the device; PA1 3. Even at the highest feasible electronic sampling rate the requirement for multiple complete samplings of the array limits the maximum linear velocity of the scanning head with respect to the medium to values that are not compatible with ergonomic considerations; in particular, for some individuals the manipulation of the device may appear to be too slow and deliberate for comfortable utilisation; PA1 4. The cost of a photodetector matrix is high compared with that of a simple linear column array, and the logic associated with choice of best image is costly and not always successful.
Another scheme that has been tried is to roll a wheel along the scanned medium, which wheel drives a miniature tachometer that in turn controls the column scan rate. This also fails for ergonomic reasons and has not been found to be a commercial success.